This invention relates to apparatus for processing material or product with circulated conditioned air.
Exemplary of apparatus which may incorporate the invention is a so-called direct-fired kiln or dryer. In kiln apparatus of this description, air is drawn out of the dryer and in a separate chamber conditioned as by heating it. The air is then returned and distributed within the dryer after heating. While the invention is described hereinbelow in connection with a direct-fired dryer or kiln, and embodies features specifically adapting the invention for this type of equipment, the invention is in a broader sense applicable to other types of equipment, such as apparatus utilizing heated air for the sterilizing or sanitizing of product by killing insect or organism growth, and in systems relying on the dehumidification of air withdrawn from a chamber with the air in the dehumidification being conditioned by lowering its humidity.
In conventional apparatus where air is conditioned as by heating it in a region outside the dryer or kiln chamber, air is withdrawn from the dryer chamber and thence passes through equipment such as a burner chamber where the air is heated. In a direct-fired system, the heated air together with products of combustion are then returned to the dryer chamber. In a dehumidification-type of heat system the air, after withdrawal from the kiln chamber, may be passed through a coil of a heat pump, with water condensed from the humid air stream and air after dehumidification returned to the dryer chamber. Within the dryer chamber, the air may be circulated with a power-driven reversible air-mover, such as a reversible fan system. In a drying kiln for lumber, air is caused to move with the air-mover first in one direction through the lumber load, and then after a period of time in the opposite direction through the load, with reversing of air movement tending to produce more uniform results. Typically, air has been removed from the kiln from a region which is the same irrespective of the direction of circulated air movement.
With such a construction, a number of problems arise. For instance, air at the opening in the kiln where air is withdrawn will have a different temperature depending upon which direction the air is being circulated through the load, such as a lumber load. Further explaining, if the circulation direction is such that the air at the withdrawal duct is for the most part preheated air which has not done any drying, the temperature will be considerably higher than is the case when the air is circulated in the opposite direction and such air has passed through the lumber and performed drying before reaching the withdrawal opening. Another factor is the change in air velocity through the load which occurs with reversal of the direction in the circulated air. If the withdrawal air opening is on the exit side of the load, the mover producing withdrawing of air is assisting the mover circulating air in the dryer chamber and producing air flow through the load. On the other hand, if the withdrawal opening from the kiln is in effect on the entering side of the load, the mover producing movement of air through the withdrawal opening tends to reduce the air velocity through the load. Also, with a system where air is withdrawn from one side of the load irrespective of air circulating direction, there tends to be energy waste, with exhausting of air occurring before the air has performed its drying function and moved through the load. Finally, any wet bulb sensor located on one side of the kiln interior will provide dramatically different readings depending upon the direction of air circulation.